1. Field of the Invention
The present invention relates to a type of device for the measurement of a temperature of an object in a vacuum environment, by which infrared rays radiated from an object are collected and introduced to a sensor through an optical fiber. Particularly, it relates to a device of the above type by which a relatively low temperature range of from below 100.degree. C. to several hundred centigrade can be measured.
2. Description of the Related Arts
The formation of a thin film formed on the surface of an object in a vacuum environment is now widely used in various fields, and the present invention is particularly applied to such a thin film formation process in a vacuum environment during for example, production of a magnetic disc, in which a surface temperature of a substrate must be controlled during the production process.
FIG. 1 illustrates a structure of a prior art device of the above type proposed by the present inventors and disclosed in Japanese Unexamined Patent Publication (Kokai) 01-107120, published on Apr. 25, 1989. This device comprises a probe 10 freely movable by the manipulation of an arm 5, and a metal bellows 9 air-tightly covering an optical fiber 8 (see FIG. 2) optically connecting the probe 10 to a sensor (not shown) incorporated in a measuring unit 12. The probe 10 is inserted to the interior of a vacuum chamber 1, in which a thin film formation is carried out on a disc-like substrate 4 rotatably supported by a shaft 6 of a holder, and the bellows 9 is extended from the measuring unit 12, positioned outside the chamber 1, to the probe 10 through an aperture in the side wall of the chamber 1. An air-tight seal between the bellows 9 and the wall of the chamber 1 is ensured by a flange 7 and an O-ring 2.
As illustrated in FIG. 2, the optical fiber 8 is enveloped with a resin coating 11 and accommodated within the bellows 9. The probe 10 is provided with a lens 13 for converging infrared rays and a protective glass 16 for protecting the lens 13 from contamination. At the front end of the probe 10 is mounted an L-shaped hood 14 having a mirror 15 detachably fixed at an inner corner thereof, for deflecting infrared rays incident thereon to the lens 13, as shown by an optical axis A.
According to the above structure, the probe 10 catches infrared rays radiated from the substrate 4, and these rays are focussed by the lens 13 on to the open end 8a of the optical fiber 8, to collect a lot of rays and improve the sensitivity of the measurement. The infrared rays travel through the optical fiber 8 to the sensor in the measuring unit 12, and thus the temperature is detected as described later.
Because the hood 14 has an L-shaped configuration, flying particles generated during the thin film formation do not reach the protective glass 16 or lens 13 but are adhered to the mirror 15, which can be replaced when the contamination by the flying particles has an adverse affect on the reflectivity. In this regard, if the mirror surface is first coated with the same material as that of the metallic thin film, the deterioration of the reflectivity of the mirror due to the deposition of the flying particles is greatly suppressed.
In the prior art temperature measurement device in which infrared rays radiated from an object to be measured are conducted through an optical fiber to a sensor, a combination of a quartz fiber and a photomultiplier or a Si-diode sensor, or a combination of a fluoride fiber and a PbS sensor, is well-known. The above combination of a fluoride fiber and a PbS sensor is used for measurements over a relatively low temperature range having a lower limit of about 120.degree. C. The reason therefor is as follows:
A perfect black body has a distribution of a spectro-radiation intensity as shown in FIG. 3, wherein the abscissa represents a wave-length (.mu.m) and the ordinate represents a radiation intensity W.sub..lambda. (Wcm.sup.-2 .mu.m.sup.-1). It will be understood that the lower the temperature of the object, the weaker the intensity of the ray radiated from the body and the greater the component of a longer wave-length. Accordingly, when the low temperature range is to be measured, it is necessary to elevate not only the transmittance of an optical fiber in the longer wave-length area but also the sensitivity of the sensor.
The losses of transmittance of a quartz fiber and a fluoride fiber (ZrF.sub.4 -BaF.sub.2 -NaF.sub.2 -AlF.sub.3) are shown in FIG. 4, wherein the abscissa represents a wave-length (.mu.m) and the ordinate represents a loss (dB/km). It will be apparent from the drawing that the quartz fiber does not permit the transmission of rays having a wave-length longer than 2.5 .mu.m, but the fluoride fiber permits the transmission of rays having a wave-length up to about 4 .mu.m. This is why the fluoride fiber can be used for the measurement of a relatively lower temperature as low as 120.degree. C. The quartz fiber is exclusively used for the measurement of a relatively high temperature range of more than 400.degree. C.
FIG. 5 illustrates the sensitivity curves of the main sensors for the detection of infrared rays operative in the room temperature range, wherein the abscissa represents a wave-length (.mu.m) and the ordinate represents a sensitivity D.sub..lambda. *. In a photomultiplier (IR21) and a Si-diode sensor, an effective wave-length range is that from visible rays to 0.9 .mu.m, and in a Pbs sensor, the same is from 1.0 .mu.m to 3.0 .mu.m. Accordingly, the above combination of a fluoride fiber and PbS sensor is suitable for the measurement of a relatively low temperature range having a lower limit of about 120.degree. C.
The temperature measurement during a thin film formation by a sputtering process by the above-described device will be explained with reference to FIG. 6, as follows.
A CoCr target 17 is struck by a shower of Ar+ ions and emits Co and Cr atoms therefrom, which then are deposited on the surface of a substrate 4 for a magnetic disc to form a thin CoCr film thereon. During the sputtering process, the temperature of the probe 10 is elevated by the effect of plasma itself, as well as a temperature rise on the target surface, which leads to an erroneous measurement of the objective temperature of the substrate surface to be detected. Especially, the temperature rise on parts of the optical system such as the mirror 15, lens 13, and protective glass 16 has a serious influence on the temperature measurement.
The provision of a cooling system in an infrared thermometer for lowering a probe temperature is disclosed in U.S. Pat. No. 3,666,949. This, however, is not intended to reduce a disturbance in the detected temperature but to protect the probe itself from damage due to a high temperature environment caused by the welding process. In U.S. Pat. No. 4,459,043, a thermometer for detecting a temperature of the interior of a turbine engine is proposed, in which a coolant gas is introduced into a probe for quenching a mirror surface. This however, also is not intended to reduce a disturbance in the detected temperature but to prevent deterioration of the mirror surface. This is because these thermometers are adapted to detect a temperature range of around 1000.degree. C., which means that the objective temperature to be measured is so much higher than that of the probe that infrared rays radiated from the probe itself do not influence the result of the measurement.
Contrary thereto, the present inventors found through experiments that, in the thin film formation process in a vacuum environment in which the surface temperature of the substrate is in a temperature range of from the room temperature to 400.degree. C., the radiation of infrared rays from the probe itself has a significant influence on the detected temperature.